Mineral acid such as sulfuric acid solutions are used to dissolve metals in cleaning operations such as in pickling or in etching of metals, to bring out designs or remove unwanted metal, in deburring operations and in electropolishing. Such acid solutions as employed usually have fast metal dissolution rates and as the metal concentration increases, the solution reaction becomes progressively slower, such that further use of the solution becomes uneconomical relatively quickly. Dumping of such a spent solution is not only a pollution hazard, but is wasteful from the standpoint of the acid as well as the metal content.
In the past, crystallization is one technique that has been used to regenerate the solutions by removal of metal values from the solution as the corresponding metal salt of the mineral acid. Specifically, the spent solutions from the metal treating operations which are carried out at elevated temperatures typically in the range of 100.degree. F. to 180.degree. F., are cooled sufficiently e.g., to a temperature in the range of 60.degree. F. to 100.degree. F., to produce a dilute crystal slurry, which subsequently is subjected to a separation step for removal and recovery of metal salt crystals. Although in some cases there is a ready market for these metal salts without any further treatment being required, perhaps except for a drying step, it is usually economically attractive to dissolve the salts in a dilute hot acid and then recover metal values as pure metal in an electrolytic plating cell from the acidic electrolyte.
Another method used with some advantage in the prior art is the direct electrolysis of acidic spent metal treatment solutions without prior crystallization, whereby metallic deposits are recovered from the cathodes and the metal depleted electrolyte is returned to the metal treatment process as regenerated treatment solution.
It is well known that all electrolytic metal recovery processes are limited, insofar as the applicable current density is concerned, by the rate with which the metal ions are diffused from the electrolyte into the liquid film layer adhering to the cathode surface. The higher the metal deposition rate on the cathode, and thus the higher the depletion rate of metal ions from the cathode film, the more this limitation affects the current efficiency and the quality of the deposits. Specifically, when the rate of ion removal from the cathode film for deposit onto the cathode surface exceeds the diffusion rate of metal ions from the electrolyte into the cathode film for replenishment, a situation is rapidly reached, where a considerable portion of the current is made available for hydrogen deposition rather than metal deposition. Under these conditions, the metal ions will plate on the cathode in an erratic manner, which leads to dentritic structure, treeing of the deposits, poorly adhering coatings of insufficient thickness, and clogging of the electrolytic cell by powdery material falling off the cathodes even to the extent of shorting current flow through the electrolyte.
It is apparent from the above that there is a maximum or "limiting" current density that can be used in any particular electrolytic system for deposits of metal of acceptable quality and thickness. Since for any given plating capacity the limiting current density directly affects the size requirements of the electrodes and, therefore, the capital cost of the entire electrolytic cell, it follows that any practical improvement which serves to increase the limiting current density without adding significant further costs would be highly desirable.
Generally, it has been recognized that the aforementioned diffusion rate decreases with increasing cathode film thickness, and, therefore, any method that results in reduction in film thickness, and, therefore, any method that results in reduction in film thickness has heretofore been considered as the best approach of increasing the limiting current density. Agitation, i.e., a rapid movement of either the electrodes or the electrolyte relative to each other is most helpful in this respect. For the agitation to be meaningful, the movement should be of relatively high velocity and act parallel to the electrode surface with a minimum of turbulance. Various methods of agitation have been suggested and used with limited success including mechanical movement by rotation or reciprocation of the electrodes in a relatively stagnant electrolyte pool, as well as movement of the electrolyte past stationary electrodes achieved by a recirculating solution pumping system, or circulation of air through the electrolyte.
However, in commercial size operations requiring large electrode surface areas and large inventories of electrolyte in the system, it has not been economically feasible to provide the power requirements necessary to achieve a sufficiently high and uniform velocity of the electrolyte past the cathode surfaces in order to substantially eliminate the cathode film layer over the entire cathode surface. Therefore, on a commercial scale, the use of agitation in any of the prior art processes has not been as beneficial as would have been desired, and there still exists a need for a practical method that will increase the diffusion rate and thereby the limiting current density.
It is therefore an object of the present invention to provide an improved process for electrodeposition of metal of acceptable quality wherein the electrodeposition can be carried out at relatively high levels of current densities.
Another object of the invention is to provide an improved process for the regeneration of spent sulfuric acid metal treatment solutions and recovery of metal values for such spent solutions.
Further objects will become apparent from a reading of the detailed description of the invention and the appended claims.